The Quebec-Labrador border is defined. It is defined in legal instruments from 1763, 1809, and 1825, as interpreted in 1927, and confirmed in 1949.

Undefined borders are, by definition, also undemarcated — border definition, of necessity, comes before demarcation; you can't demarcate a border if you don't know where (or whether) it exists.

However, undemarcated borders are often defined. Or, conversely, many defined borders are not demarcated. This is the case not only with the Quebec-Labrador border, but also with the inter-territorial borders between Yukon, the NWT, and Nunavut. (Their southern borders with the provinces, along the 60th parallel, are demarcated.) The Yukon-NWT border is set forth in the Yukon Act; the Nunavut-NWT border, in the Nunavut Act.

As a curious historical footnote, the Labrador boundary could have been surveyed in the early 1930s as a Depression-era make-work project for unemployed surveyors, but Newfoundland wasn't interested. And why would it be; after all, Newfoundland's plan for dealing with the Depression seemed to revolve around abolishing the Labrador boundary by selling the terrritory to Canada anyway!